Study On The Structure Design And Performance Of Porous Oxygen Electrode

Nowadays,energy crises and environmental issues are getting worse.Therefore,it does be of greatly practical significance to alternate the current energy structure and develop clean and pollution-free energy conversion devices.Proton exchange membrane fuel cells?PEMFCs?with high energy density and pollution-free reaction products have attracted wide attention.PEMFCs convert chemical energy into electrical energy by a cathode-side oxygen reduction reaction?ORR?and an anode-side hydrogen oxidation reaction?HOR?with the catalytic reaction of the noble-metal Pt-based catalysts.However,the sluggish oxygen reduction reaction?ORR?,not conducive to the overall energy conversion,remains a major challenge for the commercialization.In contrast with the sluggish ORR process,although the HOR kinetics process is faster,the cost of high-purity hydrogen production can not be ignored.Recently,water electrolysis as an effective approach to hydrogen production has given rise to extensive attention.Even under the catalytic reaction of state-of-the-art noble-metal-based catalyst RuO₂,however,the sluggish kinetics of anode-side oxygen evolution reaction?OER?could still remain,which seriously reduces the overall energy conversion efficiency.Based on the issues,this paper mainly focuses on the research of efficient and durable oxygen electrode catalysts.?1?Generally,traditional Pt/C has the pores mainly constructed from particle stacking,with irregular arrangement,resulting in the litmited pore volume,which is prone to flooding,but not conducive to mass transfer,leading to the poor gas transmission as well as lower utilization efficiency of noble-metal Pt.The ordered macro-mesoporous carbon was prepared via a dual-templating approach,with silica colloidal crystals and amphiphilic triblock copolymer F127 as hard and soft templates,respectively.And the highly dispersed Pt nanoparticles were loaded into the mesopores via an in-situ reduction process of ethanol,by which the ordered macromesoporous Pt/C catalyst was made.From N₂ adsorption-desorption isotherms test,together with TEM observation,the gaseous porous electrodes capacity of antiflooding peformance were constructed by macro-mesoporous and inter-contected networks,the main channel for fluid transport and storage,with the ordered and periodic arrangement as well as uniform macropore size distribution of 500 nm,increasing the pore volume to 3.5 times that of conventional Pt/C electrodes.Moreover,massive pore walls constructed by mesopores with uniform size distribution of 13.2 nm have increased the specific surface area to 4.5 times that of the conventional one,acting as the main position of the electrode reaction.In this work,we designed a special“rattle-drum”-like working electrode to estimate the antiflooding capability of the various porous catalysts in O₂-saturated 0.1 M HClO₄ electrolyte solution.As respected,the conventional Pt/C was instantaneously flooding,while the above porous electrodes could maintain continuous and stable operation.Compared to the conventional ones,single cell performances showed the output power of the porous ones were increased by 41%?45%?under full?no?humidification conditions.Thus the observation should be attributed the regulation of pore structures,unprecedentedly improving the antiflooding performance?mass transfer efficiency?.In addition,we also investigated in detail the ORR activity and durability for the macro-mesoporous Pt/C and the commercial one by electrochemical testes such as linear sweep voltammetry?LSV?and accelerated durability test?ADT?.The HRTEM test showed that the macro-mesoporous Pt/C could be ascribed to one of highly dispersed carbon-supported Pt-based catalysts with the Pt nanoparticles being supported in the mesopores.The LSV tests showed mcro-mesoporous Pt/C exhibited a slightly positive shift of half-wave potential(E_1/2,0.894 V)with that of commercial Pt/C?0.891V?,suggesting that the Pt nanoparticles loaded inside the mesopores of the dual-porosity structures could be well accessible to the reactants and have surely participated in the ORR reactions.Besides,after 10,000 CV cycles,mcro-mesoporous Pt/C catalyst well retained its ORR activity by presenting only 14 mV negative shift of half-wave potential,but the shift reached 31 mV for commercial Pt/C,indicating that the mcro-mesoporous one was therefore more electrochemically durable in ORR.Combined with TEM and XPS data,the improved ORR performance could be mainly attributed to the unique electronic interaction between macro-mesoporous carbon support and Pt nanoparticles as well as confinement effect of mesopores.?2?In view of the low density and limited utilization of active sites for traditional non-metal-based ORR catalysts,nitrogen-doped carbon?NC?,monomeric dopamine?DA?the outer nitrogen source was used to coat a layer of polymerized dopamine?PDA?on the surface of matrix ZIF8 nanoparticles,so as to obtain ZIF8@PDA composites with different degree of coating by adjusting amount of DA added.Hereafter,the high content of N-doped hollow carbon materials were prepared through annealed process for the ZIF8@PDA ones.However,when ZIF8 was solely calcined,it resulted in the same shape,solid NC dodecahedron nanoparticles.Considering the unusual observation,we then gave this hollowing process a detailed investigation of FTIR,TEM,TGA-DSG etc.and proposed a“stresses induced orientation contraction”mechanism to address it,in which the three key factors could be necessary:1)an intensive core-shell interfacial interaction;2)the distinctly higher shrinkage degree of the cores than the shells;and 3)the relatively loose core structures.In addition,the ORR activity and durability of the prerared catalysts were studied in detail by electrochemical testes such as LSV,ADT,and Zn-O₂ batteries.The LSV tests exhibited catalyst NC15-900°C owned the highest half-wave potential 0.848 V,even 10 mV more positive than that of commercial Pt/C.Besides,after 5,000 cycles of ADT,NC15-900°C also showed good durability as the E_1/2 decreases only 25 mV,but the shift got 31 mV for commercial one.Moreover,the maximal power density of the Zn-O₂ battery assembled with catalyst NC15-900°C was 270 mW cm^-2,being much higher than the value of 197 mW cm^-2 for that loaded with JM-Pt/C.Combined with BET,TEM,XPS et al.tests and LSV data,the super ORR performance of the catalyst NC15-900°C could be assigned mainly to the high absolute content of pyridinic N capacity of a high number of active sites,as well as high specific surface area and multi-level pore structures of macro-meso-micropore,beneficial to the exposure and mass transfer of the active sites on the surface of the catalyst.?3?Considering the fact that the well-controlled synthesis of doped hollow carbon-based catalysts could be difficult via a soft template method because of the presence of phase separation.With block copolymer PS-b-PEO as soft template,based on the single micelle self-assembled principle,we could prepare different heteroatom-doped hollow carbon catalysts?N-HC,S-HC,N,S-HC?.Furthermore,TEM,and N₂ adsorption-desorption tests showed that the hollow carbon material has quite similar cavity structure with a diameter ca.25 nm,and pore structure parameters of large specific surface area(583⁶41 m² g^-1),large pore volume(1.21¹.40 m³ g^-1)and the similar pore size distributions of ca.24.4 nm.Besides,electrochemical tests such as LSV and ADT were carried out to give insight into the ORR activity and durability of the obtained ones in detail.LSV tests showed the catalyst N,S-HC owned a half-wave potential 0.763 V,only less than that of state-of-the-art catalyst JM-Pt/C by 75 mV,and the former only attenuated 31 mV after 5,000 cycles of ADT.Combined with the physical characterization of TEM,XPS,N₂ adsorption-desorption and so forth tests and the corresponding LSV data,the robust ORR performance of the catalyst N,S-HC could be mainly attributed to the large specific surface area and large pore volume,N,S co-doping and its synergistic effect caused by asymmetric spin and charge density.?4?Taking the poor OER performance of single-metal Fe-based catalysts into account,a well-controlled catalyts Fe₃O₄/FeS₂-2.5 with amount of heterogeneous interface structures Fe₃O₄/FeS₂ by adjusting the mass ratio of thiourea to monodisperse Fe₃O₄ microspheres was firstly reported.Electrochemical testes,LSV and ADT et al.,were used to characterize the OER performance.The LSV tests showed that the catalyst Fe₃O₄/FeS₂-2.5 did be of a low overpotential only 253 and 306 mV to afford a current density 10 and 100 mA cm^-2,respectively,much lower than that of noble-metal-based catalyst RuO₂ and the comparative ones,and the OER polarization curves before and after the 2,000 ADT almost coincided.Combined with TEM,XRD,XPS etc.tests and LSV data,the enhanced OER performance could be well indexed to the reasons that a large number of interfacial structures Fe₃O₄/FeS₂ might as well promote charge transfer between Fe,S and O?in accordance with the result of bader charge based on DFT calculation?,as well as enforced the interactions between Fe,S,O species.Furthermore,combined with the DFT computations,the well-constructed Fe₃O₄/FeS₂ heterostructures could be of a vital effect on modulation for the adsorption free energies of oxygen-containing intermediates?*OH,*O,*OOH?to optimize their binding energies,facilitating the sluggish OER kinetics disrable for OER performance.